In the context of a clinical trial strategy seeking to identify agents that could overcome or circumvent multidrug resistance, my laboratory identified the histone deacetylase inhibitor depsipeptide as an agent in preclinical development and a substrate for Pgp-mediated efflux. Because depsipeptide is avidly transported by Pgp, and because it induces MDR-1 in the constellation of genes altered by histone acetylation, we thought it would fail in clinical development without the addition of a Pgp modulator. In the absence of Pgp, depsipeptide is active in the nanomolar range, whereas other HDAC inhibitors are less potent. Overexpression of Pgp renders cells highly resistant to depsipeptide. To prevent Pgp-mediated resistance to this agent, we reasoned that a Pgp inhibitor should be added early in clinical development. However, the agent had to be evaluated alone in the Phase I setting, and it was there that we made the serendipitous discovery that depsipeptide was highly effective in subsets of T cell lymphoma. While we have continued to be interested in our original strategy of preventing the emergence of resistance to this agent, we have pursued the use of depsipeptide as an orphan drug in T cell lymphoma, using both laboratory and clinical strategies.Our clinical trial for cutaneous and peripheral T cell lymphoma has enrolled 49 patients to date, divided into 4 cohorts. Cohort 1 includes patients with cutaneous T cell lymphoma with fewer than 2 systemic chemotherapy regimens in prior therapy. Responses in this cohort have remained at 50% with 20 patients enrolled. These responses are at times dramatic and have been very durable. As examples, one patient has received therapy for over 3 years, remaining in a partial remission. Another patient remains in complete remission off of therapy for over 1 year. Slow accrual to this trial, despite recognition of the activity of the agent led us to invite other sites to participate in the trial. Five sites were added in 2002 and we recently received IRB approval for an additional 15 sites to be added. Financial limitations, primarily related to the extensive cardiac monitoring required have hindered activation and accrual at many of these sites. The recent licensing of the agent from Fujisawa Pharmaceuticals to Gloucester Pharmaceuticals and approval of orphan drug status from the FDA should provide an opportunity for increased support of this trial. NCI CTEP and our CTB have largely pushed the development of this agent alone during a period in which Fujisawa Pharmaceuticals debated the relative merits of becoming involved in an oncology development platform. The trial has a significant translational component that has consumed a major fraction of my laboratory resources. We have developed a quantitative immunoblot assay for detecting and quantitating histone acetylation in patient samples, principally peripheral mononuclear cells as a surrogate. Results from these assays are currently being compared to pharmacokinetic data. We have also evaluated gene expression - including CD25, p21, and MDR1 by RT-PCR, finding that only MDR1 expression is induced sufficiently following depsipeptide for routine assay in patient mononuclear cells. MDR1 is also analyzed in tumor samples before therapy is initiated and then at the time of disease progression to determine whether MDR1/P-glycoprotein has an impact on responsiveness to depsipeptide. Additional studies include a Phase I trial of depsipeptide on a day 1, 3, and 5 schedule in hopes of achieving a more continuous drug effect. Accrual is ongoing, and by recent amendment Dr. Piekarz shifted into the PI position. The third dose level has been achieved. Intrapatient dose escalations are allowed and most patients have tolerated a 5 mg/m2/dose administered on days 1, 3, and 5, but no more. It is interesting that the total dose that appears to be tolerable on this schedule is only 15 mg/m2, almost identical to the 14 mg/m2 that is tolerable with each administration on a day 1, 8, and 15 schedule. This study has a focus in thyroid cancer in order to translate the observation made in Tito Fojo's laboratory that among the genes induced by depsipeptide were the genes encoding the sodium iodide symporter and thyroglobulin. Induction of these genes in thyroid cancer cells enhanced the accumulation of radioiodine in vitro 28. Such an effect in patients with thyroid cancer could lead to increased radioactive iodine uptake. The Phase I trial is ongoing, with samples collected for pharmacokinetic and pharmacodynamic analysis. PBMCs for histone acetylation determination and pre/post treatment tumor samples have been obtained for gene expression analysis. In patients with thyroid cancer (one patient per dose level and then expanding at the MTD), radioiodine imaging is allowed, along with subsequent dosing with radioiodine in the event that increased radioiodine accumulation is demonstrated. Since only patients with thyroid cancer without visible radioiodine uptake are enrolled on the study, this will probably be difficult to achieve. It is our hope that pre/post treatment biopsies will at a minimum show induction of the Na+/I- symporter at the gene expression level, in cells assayed by RT-PCR, if radioiodine uptake cannot be documented.Finally, we have for some time been interested in mechanisms of drug resistance not mediated by Pgp. This led us to the generation of cell lines with non-Pgp mediated multidrug resistance and we have begun to ask whether other mechanisms of resistance can be identified. The success of depsipeptide in CTCL has led to several obvious questions: Does Pgp confer resistance at the time of disease progression? Does Pgp confer resistance to depsipeptide in other tumor types? Could depsipeptide be safely combined with a Pgp inhibitor? Can HDI-specific resistance mechanisms be identified that are not linked to the ABC transporters?